Hi,
It looks like WordPress made a change in version 3.9 that causes the tinyMCE editor to strip out all javascript ‘on’ events. They seem to have debated fixing it and decided not to:
https://core.trac.wordpress.org/ticket/27858
If this is a one-time thing, I can only suggest a (not very good) workaround: since the editor only strips out javascript when you switch to ‘visual’ mode, you can switch the editor to ‘html’ mode, type in the javascript, and save the map. The javascript should stay in place, or at least that worked for me. But… the caveat is that every time you edit the POI after that, the editor will start up in ‘visual’ mode and try to delete the javascript again.